Method for making a mold for casting highly reactive molten masses

ABSTRACT

The invention relates to a method for producing a casting mould for casting highly reactive melts, in particular for casting titanium, titanium alloys or intermetallic titanium aluminides. Said method consists of the following steps: a contact layer ( 1 ) is produced by applying a first slicker containing a first Y 2 O 3 -powder as an essentially solid component, to a mould core, the contact layer ( 1 ) formed from the first slicker is sanded with a second Y 2 O 3 -powder containing Y 2 O 3  as the essential component. With respect to a particular efficient process, a first dry mass for producing the first slicker contains at least 75 wt. % Y 2 O 3  and as additional solid components, at least 1.0-25 wt. % of a hydraulic binder.

The invention relates to a method for making a mold for casting highly reactive molten masses, in particular forcasting titanium, titanium alloys or intermetallic titanium aluminides, as defined by the preamble of claim 1.

Such a method is known from WO 2005/039803. A slicker contains yttrium oxide, magnesium oxide and calcium oxide for the making of a contact layer or face-coat layer. Together with the water of the slicker, the magnesium oxide causes an exothermal reaction during which water evaporates and the drying time of the contact layer made from the slicker is reduced. Nothing is known from this document about the amount or the amount relationships of the oxides used.

The object of this invention is to eliminate the disadvantages as per prior art. In particular, a method for making a mold for casting highly reactive molten masses is to be specified which can be performed as simply, reproducibly and quickly as possible.

This object is solved by the features of claim 1. Useful embodiments of the invention result from the features of claims 2 to 25.

According to the invention, it is provided that a first dry mass for making the first slicker contains at least 75 wt. % of Y₂O₃ and as an additional solid component at least 1.0 to 25 wt. % of a hydraulic binder.

In the sense of the present invention, a “hydraulic binder” is understood to mean a mixture of substances which hydrates with water. The hydration products formed by this cause the solid components contained in the slicker to harden.

A particularly fast hardening of the first slicker can be achieved with the suggested composition. In particular, it is possible to apply the first slicker in a relatively very diluted state to the mold core using an injection method. This in turn provides a particularly simple and fast way to make a mold for casting highly reactive molten masses.

According to an advantageous embodiment, it is provided that the first dry mass contains less than 95 wt. % of Y₂O₃, preferably less than 90 wt. %. Surprisingly, it has been shown that the relatively expensive Y₂O₃ component which is required for the stability against a corrosive attack by highly reactive molten masses can be reduced by adding a hydraulic binder without reducing the resistance of the contact layer to the corrosive effect of highly reactive molten masses. For example, the content of Y₂O₃ in the first dry mass can be reduced to 80 or 85 wt. %. Part of the Y₂O₃, 5 to 10 wt. % for example, can be replaced by TiO₂. The content of the hydraulic binder is advantageously between 8 and 16 wt. %.

As already mentioned above, the first slicker can be applied with an injection method to the mold core using the method in accordance with the invention. This permits the making of a particularly smooth and thin contact layer as well as a particularly efficient performance of the method.

According to an advantageous embodiment, the grain band of the first powder is between 0 and 50 μm and advantageously has a medium grain size (d₅₀) in the range from 8 to 20 μm. It has been shown that a slicker with the suggested grain size distribution can be processed particularly well with the injection method on the one hand, and on the other hand, a high surface quality of the contact layer can be achieved with this.

The second Y₂O₃ powder can have a medium grain size (d₅₀) in the range from 130 to 200 μm. The suggested medium grain size of the second Y₂O₃ powder contributes to good pourability and _(.)thus makes sanding the contact layer easier.

The hydraulic binder preferably contains CaO*Al₂O₃. Such a calcium aluminate cement usefully contains 60 to 80 wt. % of Al₂O₃, preferably approximately 70 wt. % of Al₂O₃. A contact layer made by using the suggested hydraulic binder exhibits an excellent hardness. The first dry mass usefully only contains unavoidable impurities of MgO. A particularly good resistance to highly reactive molten masses, in particularly to titanium aluminide molten masses, can be achieved with this.

According to a further embodiment feature, a coating layer surrounding the layer sequence of contact layer and first sanding layer or the plurality of layer sequences can be established. The coating layer can contain MgO as an essential component. Moreover, a second dry mass for making the coating layer can contain a hydraulic binder, preferably calcium aluminate cement. Advantageously, the latter contains 60 to 80 wt. % of Al₂O₃, preferably approximately 70 wt. % of Al₂O₃. A second dry mass usefully contains at least 40 wt. % of MgO, preferably 60 to 80 wt. %, as well as at least 20 wt. % of the hydraulic binder. Moreover, the second dry mass can contain one or more of the following oxides: Fe₂O₃, SiO₂, CaO, Al₂O₃. The coating layer is essentially used for the mechanical stabilization of the contact layer. It can have a significantly greater layer thickness than the contact layer.

It has proven to be particularly useful to apply an intermediate layer sequence formed from an intermediate and a second sanding layer onto the layer sequence formed by contact and first sanding layer before the coating layer is made. The intermediate layer can be made by a second slicker applied using the injection or dipping method. The second slicker can contain a first MgO powder as the essential solid component. Moreover, the second slicker can have a hydraulic binder, preferably a calcium aluminate cement (CaO*Al₂O₃) with 60 to 80 wt. % of Al₂O₃, preferably approximately 70 wt. % of Al₂O₃. A third dry mass for making the second slicker can contain at least 50 wt. % of MgO, preferably 60 to 70 wt. % and at least 20 wt. % of the hydraulic binder. The second sanding layer is usefully made by applying a second MgO powder onto the second slicker layer.

Several such intermediate layer sequences which are formed respectively from an intermediate layer and a second sanding layer can be applied in succession onto the layer sequence made of the contact and first sanding layer. The first MgO powder can have a smaller medium grain size than the second MgO powder.

In addition, the first and/or the third dry mass/masses and/or the sanding layer/layers can contain at least one of the following oxides: CeO₂, La₂O₃, Gd₂O₃, Nd₂O₃, Ti0 ₂. The addition of other oxides of the rare earths is also possible.

The thus established intermediate layer sequence contributes to an improved thermo shock resistance of the mold. Since the intermediate layers also contain a hydraulic binder, they can also be made quickly and efficiently, particularly using the injection method. A moisture content of the contact and/or the. intermediate layer can be reduced via infrared radiation to a specified value after their application. The specified value can be in the range from 10 to 60% of residual moisture, preferably less than 20% and more than 5% of residual moisture.

Particularly with regard to the mechanical stability of the mold as well as its efficient manufacturing, it has proven to be useful that the fraction of hydraulic binder in the first dry mass is less than in the second or third dry mass. The fraction of the hydraulic binder in the second and/or third dry mass is advantageously at least 2 wt. %, preferably at least 5 wt. % greater than in the first dry mass.

Furthermore, it has proven to be useful that the first and/or second slicker has/have a viscosity of not more than 1000 mPas, preferably between 450 and 750 mPas. Slickers with such a viscosity can be processed particularly well using the injection method.

According to further embodiment features of the method, the mold core is removed by melting or burning out the material creating the mold core after the coating layer is made. The material is usefully wax or similar. A green body created after removing the mold core is usefully sintered at a sintering temperature of more than 800° C. and less than 1200° C. Using a hydraulic binder for making the mold as suggested by the invention thus also contributes to a significant reduction in the sintering temperatures.

Examples will now be used to describe the invention in more detail.

The sole figure schematically shows a partial cross sectional view through a mold according to the invention. A contact layer 1 contains 85 wt. % of Y₂O₃ and 15 wt. % of hydration products of the calcium aluminate cement, for example. Aside from unavoidable impurities, it is usefully free of MgO. Reference sign 2 designates a first sanding layer which essentially consists of a further Y₂O₃ powder having a medium grain size of approximately 150 μm. Aside from unavoidable impurities, the first sanding layer 2 is also usefully free of MgO. The contact layer 1 and the first sanding layer 2 form a layer sequence A.

An intermediate layer 3 is applied onto the first sanding layer 2 which intermediate layer 3 contains MgO as the essential component which in turn is set by the reactive product of a calcium aluminate cement. Reference sign 4 designates a second sanding layer which is made from an MgO powder. An intermediate layer sequence made of alternating layers of intermediate layer 3 and second sanding layer 4 is designated by the reference sign B. The intermediate layer sequence B is lined in the back with a coating layer 5 which contains MgO as the essential solid component which in turn is set by a hydraulic binder, preferably calcium aluminate cement.

Similar to the intermediate layer sequence B, the layer sequence A can also be created from a sequence of several contact layers 1 as well as first sanding layers 2 in alternating sequence.

EXAMPLE 1

To make the contact layer 1, a first slicker is made first whose first dry mass contains 80 to 90 wt. % of a first Y₂O₃ powder. The medium grain size d₅₀ of the first Y₂O₃ powder is usefully 10 to 15 μm. The modal value is advantageously 15 to 25 μm. The grain band is usefully in the range between 0 and 50 μm. Moreover, the first dry mass contains 10 to 20 wt. %, preferably 8 to 17 wt % of a calcium aluminate cement. Adding a suitable amount of water creates a first slicker having a viscosity in the range from 400 to 700 mPas. The first slicker is injected using the injection method onto a mold core made, for example, of wax. Then the contact layer 1 made from the first slicker is sanded with a first sanding layer 2. The first sanding layer 2 consists of a second Y₂O₃ powder which has a medium grain size in the range from 170 to 200 μm and usefully contains only unavoidable impurities. The thus created layer sequence A is then dried for a period of approximately 90 to 180 minutes or a suitable residual moisture of 10 to 30% is set.

The previously mentioned procedure of making the contact layer 1 as well as the first sanding layer 2 can be repeated multiple times, three to five times, for example. The layer sequence A can also be concluded by a contact layer 1 instead of the first sanding layer 2. It can also be that a first sanding layer which concludes the layer sequence A is essentially made of an MgO powder. This MgO powder can be made like the second Y₂O₃ powder regarding its grain size distribution as well as its medium grain size.

A second slicker is made to make one or more intermediate layers 3 of the intermediate layer sequence B. A second dry mass for making the second slicker contains, for example, 65 to 80 wt. %, preferably 65 to 70 wt. % of MgO as well as 20 to 35 wt. %, preferably 30 to 35 wt. % of a calcium aluminate cement. Adding a suitable amount of water makes a second slicker whose viscosity is usefully set so that it enables a coating using the conventional dipping method. To make the intermediate layer 3, the layer sequence A applied onto the mold core is coated with the second slicker using the dipping method. Afterwards, the second sanding layer 4 is applied which is essentially created from an MgO powder having ^(.)a grain size in the range from 0.1 to 2.0 mm. After making the second sanding layer 4, the thus created intermediate layer sequence B is dried for a period of 15 to 45 minutes or a suitable residual moisture of 10 to 30% is set. Additional intermediate 3 and second sanding layers 4 can then be applied in the same way. Finally, the intermediate layer sequence B can be dried for a period of 30 to 100 minutes.

To make the coating layer 5, 60 to 80 wt. %, preferably 70 to 80 wt. % of MgO, 20 to 40 wt. %, preferably 20 to 30 wt. % of calcium aluminate cement as well as water and auxiliary substances are used to make a still flowable mass. The coated mold core is then placed in a mold and surrounded with the flowable mass. The mold is removed after the flowable mass has dried. The mold core is removed by increasing the temperature. Then the thus created green body of the mold is sintered at a temperature in the range from 1000 to 1200° C., preferably in the range from 1100 to 1200° C.

EXAMPLE 2

In this case, a first dry mass for making the first slicker contains 60 to 70 wt. % of Y₂O₃ and 10 to 20 wt. % of CeO₂. The grain band of the mixture is between 0 and 50 μm. Moreover, the first dry mass contains 10 to 20 wt. % of the calcium aluminate cement. The first dry mass is mixed with water so that a first slicker having a viscosity in the range from 300 to 600 mPas is created.

Otherwise, the method is performed as discussed in example 1.

The drying of the layer sequence A as well as the intermediate layer sequence B can also be supported with infrared radiation. For this purpose, the coated mold core can be led through a drying chamber in which the air temperature is in the range from 30 to 60° C.

In addition to the previously mentioned components, the first and/or second slicker can also contain conventional auxiliary substances, in particular the organic auxiliary substances in the usual amounts. 

1. Method for making a mold for casting highly reactive molten masses, in particular for casting titanium, titanium alloys or intermetallic titanium aluminides, with the following steps: making a contact layer (1) by applying a first slicker onto a mold core, which first slicker contains a first Y₂O₃ powder as an essential solid component, making a first sanding layer (2) on the contact layer (1) by sanding the contact layer (1) with a second Y₂O₃ powder containing Y₂O₃ as the essential component, characterized in that a first dry mass for making the first slicker contains at least 75 wt. % of Y₂O₃ and as a further solid component at least 1.0 to 25 wt. % of a hydraulic binder.
 2. Method as defined in claim 1, wherein the first dry mass contains less than 90 wt. % of Y₂O₃.
 3. Method as defined in claim 1, wherein the first slicker is applied onto the mold core using the injection method.
 4. Method as defined in claim 1, wherein a grain band of the first Y₂O₃ powder is in the range from 0 to 50 μm and advantageously has a medium grain size (d₅₀) in the range from 8 to 20 μm.
 5. Method as defined in claim 1, wherein the second Y₂O₃ powder has a medium grain size (d₅₀) in the range from 130 to 200 μm.
 6. Method as defined in claim 1, wherein the hydraulic binder is a calcium aluminate cement.
 7. Method as defined in claim 1, wherein a coating layer surrounding a layer sequence (A) of contact (1) and first sanding layer (2) is made.
 8. Method as defined in claim 1, wherein the coating layer (5) contains MgO as the essential component.
 9. Method as defined in claim 1, wherein a second dry mass for making the coating layer (5) contains a hydraulic binder, preferably calcium aluminate cement.
 10. Method as defined in claim 1, wherein the second dry mass contains at least 40 wt. %, preferably at least 60 wt. % of MgO as well as at least 20 wt. % of the hydraulic binder.
 11. Method as defined in claim 1, wherein the second dry mass contains one or more of the following oxides: Fe₂O₃, SiO₂, CaO, Al₂O₃.
 12. Method as defined in claim 1, wherein before making the coating layer (5), an intermediate layer sequence (B) formed from an intermediate (3) and a second sanding layer (4) is applied onto the layer sequence (A) formed from the contact (1) and first sanding layer (2).
 13. Method as defined in claim 1, wherein the intermediate layer (3) is made by a second slicker applied using the injection method.
 14. Method as defined in claim 1, wherein the second slicker contains a first MgO powder as the essential solid component.
 15. Method as defined in claim 1, wherein the second slicker contains a hydraulic binder, preferably calcium aluminate cement.
 16. Method as defined in claim 1, wherein a third dry mass for making the second slicker contains at least 50 wt. % of MgO and at least 20 wt. % of the hydraulic binder.
 17. Method as defined in claim 1, wherein the second sanding layer (4) is made by applying a second MgO powder onto the intermediate layer (3).
 18. Method as defined in claim 1, wherein the first and/or third dry mass/masses and/or the sanding layer/layers contains/contain at least one of the following oxides: CeO₂, La₂O₃, Gd₂O₃, Nd₂O₃, TiO₂.
 19. Method as defined in claim 1, wherein a moisture content of the contact layer (1) and/or the intermediate layer (3) is reduced using infrared radiation to a specified value after their application.
 20. Method as defined in claim 1, wherein the specified value is in the range of 10 to 60% of residual moisture, preferably less than 20% residual moisture.
 21. Method as defined in claim 1, wherein the fraction of the hydraulic binder in the first dry mass is less than in the second or third dry mass.
 22. Method as defined in claim 1, wherein the fraction of the hydraulic binder in the second and/or third dry mass is by at least 2 wt. %, preferably by at least 5 wt. % greater than in the first dry mass.
 23. Method as defined in claim 1, wherein the first and/or second slicker has/have a viscosity of not more than 1000 mPas, preferably between 450 and 750 mPas.
 24. Method as defined in claim 1, wherein after making the coating layer (5), the mold core is removed by melting and burning out the material forming the mold core
 25. Method as defined in claim 1, wherein a green body formed after removing the mold core is sintered at a sintering temperature of more than 800° C. and less than 1200° C. 